Require AuthenticatorSelection's residentKey for registration

[maxlgu]

In 1.2.1 Registration, we should specify that AuthenticatorSelection’s's residentKey is a required property.

[maxlgu]

@rsolomakhin , could you add some references to support this issue?

[rsolomakhin]

Here is a link to the discoverable credential / resident key field values: https://w3c.github.io/webauthn/#dom-authenticatorselectioncriteria-residentkey. I could not find any information in this repository that mentions that Chrome requires it. That's a bit strange. @stephenmcgruer - where should this information be added?